Image forming apparatus performing calibration, and control method therefor

ABSTRACT

An image forming apparatus that enhances measurement accuracy of a pattern image and improves quality of a printed image. Light emitted from a light emission unit based on a first measurement condition is reflected from an image bearing member, and first information is generated based on a measurement result of the image bearing member. Second information is determined based on the first measurement condition, the first information, and a second measurement condition. Light is emitted based on the first measurement condition when a first measurement image is measured, and an image forming condition is generated based on a measurement result of the first measurement image and the first information. Light is emitted based on the second measurement condition when a second measurement image is measured, and the image forming condition is generated based on a measurement result of the second measurement image and the second information.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a calibration technique of an imageforming apparatus.

Description of the Related Art

Generally, an image forming apparatus performs calibration to correctdeviation between target density and density of an actually printedimage owing to a change of an environment around the apparatus orsecular changes of parts of the apparatus. It is important to reducedeviation of density also in a color image forming apparatus because acolor balance (what is called a color tone) varies when image density ofeach color shifts. In calibration, a pattern image formed on an imagebearing member is measured. Then, image forming conditions are adjustedso that density of an output image becomes target density. It should benoted that the image forming conditions include an exposure, developingbias, and a γ correction table. The exposure and the developing bias arecontrolled in order to correct the maximum density. The γ correctiontable is generated in order to correct a gradation characteristic of animage.

An optical sensor method that is mainly used by the above-describedimage density control is roughly divided into two types including anirregular reflection type and a specular reflection type. An opticalsensor of the specular reflection type detects specular reflection lightwith a light receiver for detecting specular reflection light that isarranged opposite to a light source with respect to a normal line of anirradiated surface. On the other hand, an optical sensor of theirregular reflection type detects diffused light from the light sourcewith a light receiver for detecting irregular reflection light.

A pattern image is formed on an image bearing member, for example. Sincecarbon black is generally distributed into an image bearing member toadjust resistance, the image bearing member has high smoothness andglossiness. Then, the color of the image bearing member is black or deepgray. Since the sensor of the specular reflection type measures thereflected light from the image bearing member, density is measurableeven if black toner is used. When image density is controlled, an imageforming condition is generated on the basis of a measurement result ofthe pattern image and a measurement result of the image bearing member.

An image forming apparatus disclosed in Japanese Laid-Open PatentPublication (Kokai) No. 2003-156888 (JP 2003-156888A) detectsintensities of two reflected light components including the regularreflection component and irregular reflection component, comparesdensity of the surface of an image bearing member and density of a tonerimage, and achieves calibration on the basis of an image formingcondition obtained by a comparison calculation. The image formingapparatus of this publication controls a light source so as to keep thesame irradiation light amount in both of a case where the density of thesurface of the image bearing member is obtained and a case where thedensity of the toner image that is formed at the same position of theimage bearing member is obtained.

Incidentally, it is important to make the irradiation light amountbecome proper in order to stabilize a detection accuracy. When theirradiation light amount is too high, a reflected light amount increasestoo much and an output value of a light receiving element is saturated,which disable correct detection of density. On the other hand, when anirradiation light amount is too low and a reflected light amountdecreases too much, output variation of the light receiving element inresponse to density variation of a pattern image becomes small, whichenlarges an error of a value that is converted from the density.

A pattern image used in gradation correction control is generally formedwith a plurality of gradations. Since variation of the output of thelight receiving element in response to variation of gradation of thepattern image becomes small in a high-density area of the pattern image,an error of a value that is obtained by converting the density of adetection result in the high-density area becomes large under a lowirradiation light amount. On the other hand, when the irradiation lightamount is always high, the reflected light amount from a low-densityarea of the pattern image or from the image bearing member becomes solarge that the output value of the light receiving element is saturated,which disturbs correct detection of the density. Namely, it wasdifficult to accurately detect density of a pattern image in a widedensity range.

Moreover, development-contrast compensation needs a high-density patternimage in general. An error becomes large when the high-density patternimage is measured with a low irradiation light amount. Accordingly, whenthe pattern image for the development-contrast compensation and thepattern image for the gradation correction are measured with the sameirradiation light amount, density determination accuracy of the patternimage for the development-contrast compensation becomes low.Accordingly, it is important to set up the irradiation light amountappropriately in order to raise the density determination accuracy ofthe pattern image and to improve quality of a printed image.

SUMMARY OF THE INVENTION

The present invention provides a technique that sets up an irradiationlight amount appropriately to improve measurement accuracy of a patternimage and improves quality of a printed image.

Accordingly, a first aspect of the present invention provides an imageforming apparatus that forms an image on a sheet, the image formingapparatus includes an image bearing member, an image forming unitconfigured to form an image on the image bearing member, a lightemission unit, a measurement unit configured to measure reflected lightfrom a measurement image formed on the image bearing member, and acontroller configured to control the image forming unit to formmeasurement images, to control the light emission unit to emit light, tocontrol the measurement unit to measure reflected light from themeasurement images, and to generate an image forming condition based onmeasurement results of the measurement images and information related toa measurement result of the image bearing member. The controllercontrols the light emission unit to emit light based on a firstmeasurement condition, and controls the measuring unit to measure thereflected light from the image bearing member, and determines firstinformation corresponding to the first measurement condition based onthe measurement result of the image bearing member. The controllerdetermines second information corresponding to a second measurementcondition based on the first measurement condition, the firstinformation, and the second measurement condition. Light intensitycorresponding to the second measurement condition is more than lightintensity corresponding to the first measurement condition. Thecontroller controls the light emission unit to emit light based on thefirst measurement condition in a case where the measurement unitmeasures a first measurement image based on the first measurementcondition, and generates the image forming condition based on ameasurement result of the first measurement image and the firstinformation. The controller controls the light emission unit to emitlight based on the second measurement condition in a case where themeasurement unit measures a second measurement image based on the secondmeasurement condition, and generates the image forming condition basedon a measurement result of the second measurement image and the secondinformation.

Accordingly, a second aspect of the present invention provides a controlmethod for an image forming apparatus that forms an image on a sheet,the control method including controlling an image forming unit to form ameasurement image on an image bearing member, controlling a lightemission unit to emit light, controlling a measurement unit to measurereflected light from the measurement image, and generating an imageforming condition based on a measurement result of the measurement imageand information related to a measurement result of the image bearingmember. The measurement unit measures light emitted from the lightemission unit based on a first measurement condition and is reflectedfrom the image bearing member, and first information corresponding tothe first measurement condition is generated based on the measurementresult of the image bearing member. Second information corresponding toa second measurement condition is determined based on the firstmeasurement condition, the first information, and the second measurementcondition. Light intensity corresponding to the second measurementcondition is more than light intensity corresponding to the firstmeasurement condition. The light emission unit emits light based on thefirst measurement condition in a case where the measurement unitmeasures a first measurement image based on the first measurementcondition, and the image forming condition is generated based on ameasurement result of the first measurement image and the firstinformation. The light emission unit emits light based on the secondmeasurement condition in a case where the measurement unit measures asecond measurement image based on the second measurement condition, andthe image forming condition is generated based on a measurement resultof the second measurement image and the second information.

According to the present invention, an irradiation light amount is setup appropriately, which improves the measurement accuracy of a patternimage and improves the quality of the printed image.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view showing a configuration of a printer of an imageforming apparatus according to a first embodiment of the presentinvention.

FIG. 2 is a block diagram schematically showing an image processing unitand related elements of the image forming apparatus shown in FIG. 1.

FIG. 3 is a schematic view showing a configuration of a pattern sensorin the image forming apparatus shown in FIG. 1.

FIG. 4 is a flowchart showing a process of gradation correction controlexecuted by the image forming apparatus shown in FIG. 1.

FIG. 5 is a view showing an example of a pattern image formed on anintermediate transfer belt of the image forming apparatus shown in FIG.1.

FIG. 6A is a graph showing measurement results of the pattern image inFIG. 5, and FIG. 6B is a graph showing relations between density valuesof the pattern image in FIG. 5 and detected voltages.

FIG. 7 is a graph showing a relation between the sensor detectionvoltage output in a case where the intermediate transfer belt ismeasured and a light amount setting in the image forming apparatus shownin FIG. 1.

FIG. 8A and FIG. 8B are tables showing one-round profiles of theintermediate transfer belt that are shown by reflected light outputs inthe image forming apparatus shown in FIG. 1, and FIG. 8C is a graphshowing relations between measuring positions and reflected lightoutputs in the one-round profiles.

FIG. 9 is a flowchart showing a one-round profile obtaining processexecuted by the image forming apparatus shown in FIG. 1.

FIG. 10 is a graph showing of a relation defined in a density conversiontable in the image forming apparatus shown in FIG. 1.

FIG. 11 is a graph showing an example of a γLUT in the image formingapparatus shown in FIG. 1.

FIG. 12A is a view showing the measurement accuracies in a case wheremeasurement images of various gradations of the pattern image aremeasured with a low irradiation light amount in comparison with themeasurement accuracies in a case where the measurement images aremeasured with a high irradiation light amount with the image formingapparatus shown in FIG. 1. FIG. 12B is a view showing the measurementaccuracies of the measurement images of various gradations of thepattern image in the first embodiment in comparison with the measurementaccuracies of the measurement images of various gradations of thepattern image in the low irradiation light amount.

FIG. 13 is a view showing an example of a pattern image fordevelopment-contrast compensation in an image forming apparatusaccording to a second embodiment of the present invention.

FIG. 14 is a flowchart showing a process of density correction controlexecuted by the image forming apparatus according to the secondembodiment.

FIG. 15 is a graph showing a relation between the density value of thepattern image and the development contrast in the image formingapparatus according the second embodiment.

FIG. 16 is a view showing comparison of the measurement accuracies incases where the pattern image of the same gradation is measured with thelow and high irradiation light amounts.

DESCRIPTION OF THE EMBODIMENTS

Hereafter, embodiments according to the present invention will bedescribed in detail with reference to the drawings.

FIG. 1 is a view showing a configuration of a printer of an imageforming apparatus according to a first embodiment of the presentinvention. The image forming apparatus is a color image formingapparatus (printer) of an electrophotographic system. The printer 10 hasfour stations for forming images of four colors including yellow (Y),magenta (M), cyan (C), and black (K).

The printer 10 is provided with laser light sources 24Y, 24M, 24C, and24K, photosensitive drums 22Y, 22M, 22C, and 22K, electrostatic chargers23Y, 23M, 23C, and 23K, and development devices 26Y, 26M, 26C, and 26Kcorresponding to the four colors. Moreover, the development devices 26Y,26M, 26C, and 26K are respectively provided with sleeves 26YS, 26MS,26CS, and 26KS.

The photosensitive drums 22Y, 22M, 22C, and 22K are constituted byapplying an organic photoconductive layer to a periphery of an aluminumcylinder, and are rotated by driving force of a drive motor (not shown).This drive motor rotates the photosensitive drums 22Y, 22M, 22C, and 22Kcounterclockwise in FIG. 1 in response to an image forming operation.The laser light sources 24Y, 24M, 24C, and 24K emit light beamsaccording to digital signals from a reader (not shown) to formelectrostatic latent images on the photosensitive drums 22Y, 22M, 22C,and 22K that were uniformly electrified by the electrostatic chargers23Y, 23M, 23C, and 23K, respectively. The electrostatic latent imagesformed on the photosensitive drums 22Y, 22M, 22C, and 22K are visualizedas toner images by the development devices 26Y, 26M, 26C, and 26K.

An intermediate transfer belt 27, which is an intermediate transfermedium and also an image bearing member, rotates clockwise insynchronization with rotations of the photosensitive drums 22Y, 22M,22C, and 22K. Moreover, the intermediate transfer belt 27 contacts withthe photosensitive drums 22Y, 22M, 22C, and 22K. The toner images on thephotosensitive drums 22Y, 22M, and 22C and 22K are transferred to theintermediate transfer belt 27 at contact portions. The intermediatetransfer belt 27 is a monolayer resin conveyor belt that is made frompolyimide with a circumference of 895 mm. Moreover, a proper quantity ofcarbon particulates are distributed into the resin for adjustingresistance of the belt. Accordingly, the intermediate transfer belt 27is black, and has high smoothness and glossiness. Rotational speed ofthe intermediate transfer belt 27 is set up in 246 mm/sec as well asprocess speed.

An HP (home position) mark 43 attached to the intermediate transfer belt27 is detected by an HP detection sensor 44 by every one round of theintermediate transfer belt 27. A phase of the intermediate transfer belt27 can be specified by the elapsed time from the timing at which the HPmark 43 was detected. This allows adjustment of the relative relationbetween the position of a pattern image P1 (FIG. 5) formed on theintermediate transfer belt 27 and the phase of the intermediate transferbelt 27. Moreover, an optical pattern sensor 41 for measuring thepattern image P1 is arranged opposite to the intermediate transfer belt27 at a downstream side of the transfer section, i.e., the contactposition between the photosensitive drum 22K and the intermediatetransfer belt 27.

The multicolor toner image supported by the intermediate transfer belt27 is transferred to a sheet 21 that is conveyed from a feeding unit 11and is conveyed while being nipped between the intermediate transferbelt 27 and a roller of a transfer unit 28. After that, a heat fixingprocess is applied to the toner image transferred to the sheet 21 by aheating roller 31 and a pressure roller 32 of a fixing unit 30. When thesheet 21 to which the toner image was fixed is ejected from the fixingunit 30, the sheet 21 is detected by an ejection sensor 42 and isejected out of the apparatus.

FIG. 2 is a block diagram schematically showing the image processingunit and related elements of the image forming apparatus 10 shown inFIG. 1. The image foiling apparatus 10 has an image processing unit 50that processes an image read by the reader (not shown) in addition to aCPU 51. A timer 55 is controlled by the CPU 51 and used for various timemanagement. The CPU 51 integrally controls sections of the image formingapparatus according to a control program stored in a ROM 52 while usinga RAM 53 as a work memory. The reader is provided with a CCD sensor 501that converts an image of a read original into an electrical signal.This CCD sensor 501 is a color sensor with three lines of RGB. Imagesignals of R (Red), G (Green), and B (Blue) components that were outputfrom the CCD sensor 501 are input into an A/D convertor 502 of the imageprocessing unit 50.

The A/D convertor 502 converts the image signal of each color into 8-bitdigital image data after performing gain adjustment and offsetadjustment. The digital image data output from the A/D convertor 502 isinput into a shading correction module 503. The shading correctionmodule 503 corrects sensitivity dispersion between pixels of the CCDsensor 501, dispersion in the light amount of a document illuminationlamp, etc. for every color while using signals obtained by reading astandard white plate. An input gamma correction module 504 corrects eachof the RGB signals that were input from the shading correction module503 while using a one-dimensional look-up table (LUT) so that luminancehas a linear relation with the signal. An input direct mapping module505 converts the RGB signals that were input from the input gammacorrection module 504 into in-device RGB signals while using athree-dimensional LUT in order to unify a color space. Thethree-dimensional LUT can be used to convert a reading color spacedependent on spectral characteristics of RGB filters of the CCD sensor501 into a standard color space like an sRGB color space so as to absorbvarious characteristics, such as the sensitivity characteristics of theCCD sensor 501 and the spectral characteristics of the illuminationlamp.

The data output from the input direct mapping module 505 is input into asampling module 506. The sampling module 506 discretely samples pixelswithin a designated rectangular area measure and generates a histogramof luminance in order to measure a ground of an original. This histogramis used for ground elimination at a time of printing. A backgroundelimination module 507 nonlinearly converts the RGB image data toeliminate measured values of the ground of the original on the basis ofthe result of the sampling module 506. An output direct mapping module508 converts the RGB image data input from the background eliminationmodule 507 into CMYK image data. In this conversion, the output directmapping module 508 generates four-dimensional data of C (Cyan), M(Magenta), Y (Yellow), and K (Black) from the RGB three-dimensional datausing a look-up table. An output gamma correction module 509 correctsdensity values of the CMYK image data input from the output directmapping module 508 so as to obtain a proper output image according tothe printer. The output gamma correction module 509 has a role ofkeeping linearity of input-and-output image data that is different forevery image forming process on the basis of a one-dimensional LUT(hereinafter referred to as a γLUT) of CMYK stored beforehand. This γLUTof CMYK is updated at a timing at which a newly generated γLUT was sentto the output gamma correction module 509.

A halftone processing unit 510 is able to select an image formingprocess from among different types of image forming processes(screening) and to apply the selected process to the image data inputfrom the output gamma correction module 509. Generally, an image formingprocess of an error diffusion system that hardly causes moire is usedfor a copy operation, and an image forming process of a multiple-valuescreen system using a dither matrix etc. is used for a print operationin consideration of a gradation, stability, and reproducibility of acharacter or a thin line. The former is a correcting method that weightsa target pixel and its peripheral pixels with an error filter bydistributing the errors in the multi-valuing while keeping the number ofgradations. On the other hand, the latter is a method that sets upmultiple thresholds of the dither matrix to express half gradationsartificially. In the first embodiment, the components of CMYK areindependently converted, and the low number of lines (rough lines) andthe high number of lines (fine lines) are switchable.

FIG. 3 is a schematic view showing a configuration of the pattern sensor41 in the image forming apparatus shown in FIG. 1. The pattern sensor 41is a specular reflection type, and has a light source (light emissionunit) 411 and a light receiver 412. The light source 411 is an LED, forexample, and the light receiver 412 is a photodiode, for example.Furthermore, the pattern sensor 41 has an IC 413 that controls anemission light amount (light intensity) of the light source 411 as oneof irradiation conditions (measurement conditions). The light source 411is installed at an angle of 45 degrees to the normal line of theintermediate transfer belt 27, and irradiates the intermediate transferbelt 27. The light receiver 412 is installed at a position that issymmetrical to the light source 411 about the normal line of theintermediate transfer belt 27. The light receiver 412 receives thespecular reflection light from the intermediate transfer belt 27 or atoner image in an irradiated area and outputs a value corresponding tothe light receiving result (reflected light intensity, reflected lightamount). FIG. 3 shows a case where the pattern image P1 passes through ameasurement area of the pattern sensor 41. It should be noted that adetectable range of 0.0 [V] through 5.0 [V] is an output voltage rangeof the light receiver 412.

Incidentally, a density correction control (calibration) is achieved bycontrolling image forming conditions. Generally, the density correctioncontrol is divided roughly into two kinds including Dmax control thatadjusts development contrast by changing electrifying bias, developmentbias, laser exposure intensity, etc. and gradation correction controlthat corrects input image data using an LUT. Although the gradationcorrection control is described as an example of the density correctioncontrol in the first embodiment, the present invention is not limited tothis. The CPU 51 generates the γLUT on the basis of the measurementresult of the pattern image P1 by the pattern sensor 41 in order toobtain an ideal gradation characteristic of the image forming apparatus.The γLUT applied to the output gamma correction module 509 is equivalentto one of the image forming conditions for the printer 10 to form animage.

FIG. 4 is a flowchart showing a process of the gradation correctioncontrol. The process of this flowchart is achieved when a program storedin the ROM 51 is developed to the RAM 53 and the CPU 51 runs theprogram.

First, the CPU 51 determines whether the number of sheets (the number ofimage formation sheets) on which images were formed by the image formingapparatus after the last gradation correction control is 100 or more instep S101. It should be noted that the number of image formation sheetsis always counted with a counter. When the number of image formationsheets is less than 100, the CPU 51 continues a regular image formingoperation (step S105) and finishes the process in FIG. 4. When thenumber of image formation sheets is 100 or more, the process proceeds tostep S102. It should be noted that the determination process in the stepS101 is not limited to the above-mentioned example. For example, the CPU51 may execute the gradation correction control in a case where aconsumption amount of developer including toner exceeds a predeterminedamount. Moreover, for example, the CPU 51 may execute the gradationcorrection control in a case where an environment condition that isdetected by an environment sensor provided in the image formingapparatus is a predetermined condition. Moreover, for example, the CPU51 may execute the gradation correction control in a case whereoperating time of the image forming apparatus after the last gradationcorrection control exceeds predetermined time. Moreover, for example,the CPU 51 may execute the gradation correction control in response to auser's instruction from an input device (not shown).

In step S102, the CPU 51 controls the printer 10 to form the patternimage P1 (FIG. 5) on the intermediate transfer belt 27. In step S103,the CPU 51 measures the pattern image P1 with the pattern sensor 41,determines the density of the pattern image P1 on the basis of themeasurement result, and obtains density data. A method for determiningthe density of the pattern image P1 will be described in detail below.In step S104, the CPU 51 generates the γLUT for correcting input imagedata on the basis of the obtained density data. Then, the γLUT in theoutput gamma correction module 509 is updated to the newly created γLUT.The method for generating the γLUT will be mentioned later. After that,input image data is corrected with the updated γLUT and an image isformed according to the corrected image data in the regular imageformation in the step S105,

FIG. 5 is a view showing an example of the pattern image P1. The patternimage P1 is formed on the intermediate transfer belt 27 in the firstembodiment. The pattern image P1 is a group of measurement images eachof which is a square with a one-side of 25 mm. It should be noted thatthe pattern image P1 is enough to be formed on an image bearing member,and may be formed on the photosensitive drum 22Y, 22M, 22C, or 22K. Forexample, when the pattern sensor 41 measures a pattern image formed onthe photosensitive drum 22Y, for example, the pattern sensor 41 isenough to be arranged opposite to the photosensitive drum 22Y on whichmeasurement images are formed.

An arrow in FIG. 5 indicates a rotational direction of the intermediatetransfer belt 27. Eight measurement images are formed for each of Y, M,C, and K in the pattern image P1 while changing an image printing rate(a density gradation) in eight steps. The thirty-two measurement imagesin total are arranged in the rotational direction (circumferentialdirection) of the intermediate transfer belt 27. It should be noted thatthe irradiation light amount by the light source 411 varies depending onthe measurement images of the pattern image P1 in the first embodiment.That is, the irradiation light amount is predetermined corresponding tothe gradation of each of the measurement images of the pattern image P1.The printing rates (gradations) of the measurement images in the patternimage P1 and the corresponding irradiation light amounts are set up asfollows.

Y1, M1, C1, K1: Printing rate 12.5%, Light amount L1

Y2, M2, C2, K2: Printing rate 25.0%, Light amount L1

Y3, M3, C3, K3: Printing rate 37.5%, Light amount L2

Y4, M4, C4, K4: Printing rate 50.0%, Light amount L2

Y5, M5, C5, K5: Printing rate 62.5%, Light amount L2

Y6, M6, C6, K6: Printing rate 75.0%, Light amount L2

Y7, M7, C7, K7: Printing rate 87.5%, Light amount L2

Y8, M8, C8, K8: Printing rate 100.0%, Light amount L2

The density determination of the pattern image P1 in the step S103 inFIG. 4 will be further described with reference to FIG. 6A, FIG. 6B, andFIG. 7. The density of the pattern image P1 is determined on the basisof the light receiving result of the reflected light from the patternsensor 41. That is, the density is determined on the basis of thereflected light output corresponding to the light receiving result ofthe reflected light from the pattern image P1 in an area of theintermediate transfer belt 27 and the reflected light outputcorresponding to the light receiving result of the reflected light fromthe intermediate transfer belt 27 in the same area in which the toner isnot attached.

The irradiation light amount is limited up to a value at which thereflected light amount falls within the detectable range of the sensorin a case where the pattern image P1 and the intermediate transfer belt27 are measured with the same irradiation light amount irrespective ofthe density (gradation) of the pattern image P1. That is, theirradiation light amount is determined on the basis of the measuredvalue (reflected light output) of the intermediate transfer belt 27where the reflected light output is maximized. Against this, a profile(surface data) of the intermediate transfer belt 27 that is used for areflected light output correction process is found by calculationinstead of measurement in a case where the irradiation light amount isL2 that makes the measured value of the intermediate transfer belt 27 beoutside the detectable range of the sensor in the first embodiment.Low-density measurement images of the pattern image P1 that largelychange the sensor output with respect to the density variation aremeasured with the low irradiation light amount L1 (a first measurementcondition). High-density measurement images are measured with the highirradiation light amount L2 (a second measurement condition).

FIG. 6A is a graph showing results of measuring the pattern image P1with three different irradiation light amounts (small, medium, andlarge). A horizontal axis denotes time and a vertical axis denotesdetected voltage. FIG. 6B is a graph showing relations between thedensity value of the pattern image P1 and the detected voltage for everyirradiation light amounts. A horizontal axis denotes the density valueof the pattern image P1 and a vertical axis denotes the detectedvoltage. FIG. 6B shows that the inclination of the detected voltage withrespect to the density value of the pattern image becomes larger as theirradiation light amount becomes higher. The larger the inclination is,the smaller the error of the value that is obtained by converting thedetected voltage into the density of the pattern image P1. Accordingly,the larger inclination means higher measurement accuracy. In the firstembodiment, the irradiation light amount at the time of measurement ischanged corresponding to a gradation range (density range) of thepattern image P1. The irradiation light amount is controlled byadjusting the voltage applied to the light source 411 in the patternsensor 41 by the IC 413.

FIG. 7 is a graph showing a relation between the sensor detectionvoltage (reflected light output) output in a case where the intermediatetransfer belt 27 is measured and a light amount setting. Generally, anemission light amount of a light emission element, such as LED,increases linearly as input voltage to the light source 411 increases. Adark current voltage L0B is a detection output in a case where the inputvoltage to the light source 411 is set to 0. The dark current voltageL0B is mainly determined by sensor characteristics. As mentioned above,the light receiver 412 involves the detectable range of 0.0 [V] through5.0 [V].

In the first embodiment, the lower irradiation light amount L1 isdetermined so that the reflected light output of the intermediatetransfer belt 27 falls within the detectable range of the light receiver412. Specifically, the irradiation light amount L1 is determined so thatthe reflected light output of the intermediate transfer belt 27 becomes4.0 [V]±0.1 [V], and the input voltage to the light source 411 forachieving the irradiation light amount L1 is denoted as L1Vin. Thehigher irradiation light amount L2 is determined so that the reflectedlight output of the intermediate transfer belt 27 becomes outside thedetectable range of the light receiver 412 (the output is saturated andsticks to 5.0 [V]), and the input voltage to the light source 411 forachieving the irradiation light amount L2 is denoted as L2Vin. The inputvoltage L2Vin shall be a fixed value. It may be found by adjustingdepending on a condition, or may be found by multiplying the inputvoltage L1Vin by predetermined times.

In the above description, it is presupposed that the irradiation lightamount L1 is set to the measurement image Y2 of which the gradation is25% and the irradiation light amount L2 is set to the measurement imageY3 of which the gradation is 37.5%. Even when a threshold of thegradation differs from the above example, the irradiation light amountL1 or L2 is selected. The threshold of the gradation that divides theirradiation light amount to be selected shall be in a range of 25%through 37.5%. Accordingly, the irradiation light amount L1 is set tomeasurement images of gradations (first gradation range) below thethreshold, and the irradiation light amount L2 is set to measurementimages of gradations (second gradation range) beyond the threshold. Thethreshold (for example, 31%) of the gradation is equal to or more than agradation (for example, 30%) in which the output of the pattern sensor41 is saturated while using the irradiation light amount L2.Accordingly, the first gradation range includes a gradation in which theoutput of the pattern sensor 41 is saturated while using the irradiationlight amount L2.

Next, measurement of a profile using each of the irradiation lightamounts will be described with reference to FIG. 8A, FIG. 8B, and FIG.8C. The CPU 51 obtains a first profile that is a reflected light outputcorresponding to a light receiving result of reflected light from theintermediate transfer belt 27 that is irradiated with the irradiationlight amount L1. That is, the first profile used in the reflected lightoutput correction process for the density determination of measurementimages (first measurement image group) in the first gradation range ofthe pattern image P1 is obtained by actual measurement. On the otherhand, the second profile used in the reflected light output correctionprocess for the density determination of measurement images (secondmeasurement image group) in the second gradation range of the patternimage P1 is determined and obtained by calculation on the basis of thefirst profile and the irradiation light amounts L1 and L2.

A reflected light output L1B(i) in FIG. 7 is an output of the sensor 41in a case where the intermediate transfer belt 27 is irradiated with theirradiation light amount L1. The first profile is a profile of thereflected light output L1B(i) for one round of the intermediate transferbelt 27. A reflected light output L2B(i) in FIG. 7 is an output of thepattern sensor 41 in a case where the intermediate transfer belt 27 isirradiated with the irradiation light amount L2. The reflected lightoutput L2B(i) is an estimated value, which is not an actual measuredvalue, because the output becomes outside the detectable range of thelight receiver 412 (more than 5.0 [V] at which the output is saturated).The second profile is a profile of the reflected light output L2B(i) forone round of the intermediate transfer belt 27.

FIG. 8A and FIG. 8B are views respectively showing tables of theprofiles of the reflected light outputs L1B(i) and L2B(i) for one roundof intermediate transfer belt 27. It should be noted that the reflectedlight outputs in FIG. 8A, FIG. 8B, and FIG. 8C are obtained byconverting the analog output voltages into digital values with an A/Dconverter (not shown). The profiles for one round of the intermediatetransfer belt are stored in the RAM 53 (FIG. 2). Each of the profilesconsists of the reflected light outputs that are read by the patternsensor 41 at measuring positions denoted by data numbers n for one roundof the intermediate transfer belt 27. The data number n=0 corresponds toa position measured by the pattern sensor 41 at an HP detection timingat which the HP mark 43 is detected.

FIG. 8C is a view showing a relation between the measuring position andthe reflected light output in each of the profiles. A horizontal axisdenotes the measuring position (data number n), and a vertical axisdenotes the reflected light output of the pattern sensor 41.

When a measuring operation for one round of the intermediate transferbelts is performed to obtain a profile, the intermediate transfer belt27 rotates without toner. In this state, the pattern sensor 41 reads oneround of the rotating intermediate transfer belt 27. The CPU 51 storesspecular reflection light outputs (sensor outputs) obtained by readinginto the RAM 53 as a profile of the intermediate transfer belt 27 forone round (hereinafter referred to as a one-round profile). Rotationalspeed of the intermediate transfer belt 27 is 246 mm/sec, and aperimeter thereof is 895 mm, and a measurement time interval of thepattern sensor 41 is 4 msec (the measurement count is 250 times/sec) inthe first embodiment. Accordingly, 910 pieces of data values areobtained from the outputs of the pattern sensor 41 as shown by thefollowing formula (1).(895/246)·250≈910  (1)

As shown in FIG. 8A and FIG. 8B, the one-round profile with each of theirradiation light amounts L1 and L2 consists of 910 pieces of continuousdata values. The formation position of the pattern image P1 on theintermediate transfer belt 27 is calculated as the data number naccording to elapsed time from the HP detection timing mentioned above.The data number n is calculated on the basis of the elapsed time T (sec)from the HP detection timing according to the following formula (2).n=T·250  (2)

The CPU 51 always manages a phase of the intermediate transfer belt 27during operations of the apparatus, and specifies a position opposed tothe pattern sensor 41. The CPU 51 manages the phase (i) of theintermediate transfer belt using the position of the HP mark as astandard (0).

Next, a method for obtaining the first profile (the one-round profile ofthe reflected light output L1B(i)) and the second profile (the one-roundprofile of the reflected light output L2B(i)) will be described withreference to FIG. 9. FIG. 9 is a flowchart showing a one-round profileobtaining process. The process of this flowchart is achieved when aprogram stored in the ROM 51 is developed to the RAM 53 and the CPU 51runs the program. This process starts in response to a user'sinstruction, for example, or may be performed periodically. It should benoted that the first and second profiles are stored in the RAM 53 andare updated to the newest one after completing the one-round profileobtaining process in FIG. 9.

In step S201, the CPU 51 measures the dark current voltage L0B whilesetting the input voltage to the light source 411 to 0, and stores themeasured result into the RAM 53. Since the dark current voltage L0B istaken into consideration to calculation of the second profile,management of the phase is unnecessary. In a case where the dark currentvoltage L0B is enough smaller than the sensor output in the irradiationlight amount L1, the dark current voltage L0B may be disregarded. Instep S202, the CPU 51 finds the input voltage L1Vin at which theemission light amount of the light source 411 becomes the irradiationlight amount L1 by adjusting the voltage applied to the light source 411in the pattern sensor 41 with the IC 413, and stores the input voltageinto the RAM 53. In step S203, the CPU 51 obtains the first profile(one-round profile of the reflected light output L1B(i)) by the methodof the actual measurement mentioned above, and stores the first profileinto the RAM 53. In step S204, the CPU 51 obtains the second profile bycalculation by calculating the reflected light output L2B(i) accordingto the following formula (3), and stores the second profile into the RAM53.L2B(i)={(L1B(i)−L0B)/L1Vin}·L2Vin+L0B  (3)

The formula (3) enables the calculation of the second profile under thecondition where the effect of the dark current voltage L0B iseliminated. Then, the process in FIG. 9 finishes. Accordingly, thesecond profile is calculated by multiplying a ratio of the valuecorresponding to the irradiation light amount L2 to the valuecorresponding to the irradiation light amount L1 to the first profile(surface data) while taking the dark current voltage L0B intoconsideration.

Next, the reflected light output correction process will be described.In the reflected light output correction process, the CPU 51 correctsthe effect of the reflected light from the intermediate transfer belt 27in the specular reflection output of the pattern image P1 by dividingthe reflected light output of the pattern image P1 by the reflectedlight output of the intermediate transfer belt 27 for every irradiationlight amount. In that case, the CPU 51 calculates so that the effect ofthe dark current voltage L0B is removed.

Specifically, the reflected light output of the pattern image P1 by thepattern sensor 41 with the irradiation light amount L1 shall be denotedby L1P(i) (a second output signal). In the density determination withthe irradiation light amount L1, the CPU 51 calculates a correctionoutput SIG(i) of the pattern image P1 according to the following formula(4).SIG(i)=(L1P(i)−L0B)/(L1B(i)−L0B)  (4)

On the other hand, the reflected light output of the pattern image P1 bythe pattern sensor 41 with the irradiation light amount L2 shall bedenoted by L2P(i). In the density determination with the irradiationlight amount L2, the CPU 51 calculates a correction output SIG(i) of thepattern image P1 according to the following formula (5).SIG(i)=(L2P(i)−L0B)/(L2B(i)−L0B)  (5)

Subsequently, the CPU 51 converts the correction output SIG(i) into adensity value DENS(i) of the pattern image P1 using a density conversiontable shown in FIG. 10. The density conversion table is beforehandstored in the ROM 52, and is generated in accordance with outputcharacteristics of the pattern sensor 41. It should be noted that thedensity conversion table may be generated and held for each of theirradiation light amounts. In the first embodiment, the pattern sensor41 measures a measurement image of the same density of the pattern imageP1 (formed in the same gradation) multiple times (for example, 10times). Then, the average of the ten density values that are convertedon the basis of the density conversion table is found as the densityvalue DENS(i) of the measurement image concerned.

Thus, the CPU 51 determines the density value DENS(i) of the patternimage P1 on the basis of the reflected light output (reflected lightamount) of the pattern image P1 and the reflected light output(reflected light amount) of the intermediate transfer belt 27. Since thedensity value DENS(i) of the pattern image P1 is the density valueobtained in consideration of the unevenness of the surface state of theintermediate transfer belt 27, the density is determined with highaccuracy by the reflected light output correction process. Furthermore,correction data is generated on the basis of the calculated result.Details of the process will be described later with reference to FIG.11. Then, the CPU 51 sends the generated correction data to the imageprocessing unit 50.

Next, a concrete example of the gradation correction control in FIG. 4will be described. The gradation correction control is performed alongthe following procedures (a1) through (d2).

Procedure (a1): When the gradation correction control is performed, theCPU 51 makes the pattern image P1 form on the intermediate transfer belt27. This is equivalent to the step S102 in FIG. 4. Then, the CPU 51measures the pattern image P1 with the pattern sensor 41 in a statewhere the pattern sensor 41 is controlled so as to switch theirradiation light amounts L1 and L2 mentioned above. That is, the CPU 51measures the measurement images Y1 and Y2 in the first gradation range(the gradation is 25% or less in the example) using the irradiationlight amount L1, and measures the measurement images Y3 through Y8 inthe second gradation range (the gradation is 37.5% or more in theexample) using the irradiation light amount L2.

Procedure (b1): The CPU 51 specifies the reflected light output of theposition on the intermediate transfer belt 27 corresponding to theformation position of the pattern image P1 on the basis of theirradiation light amount at the time of measuring the pattern image andthe formation position of the pattern image P1. In this time, the CPU 51specifies the reflected light output of the position on the intermediatetransfer belt 27 corresponding to each of the formation positions fromthe first (second) profile when the irradiation light amount is L1 (L2).

Procedure (c1): The CPU 51 determines the density of the pattern imageP1 using the reflected light output of the pattern image P1 and thereflected light output of the intermediate transfer belt 27.Specifically, the CPU 51 calculates (determines) the density valueDENS(i) about the measurement images of the pattern image P1 in thefirst gradation range on the basis of the value obtained from the firstprofile and the reflected light output of the pattern image P1 in thecase where the irradiation light amount L1 is used. Moreover, the CPU 51calculates (determines) the density DENS(i) about the measurement imagesof the pattern image P1 in the second gradation range on the basis ofthe value obtained from the second profile and the reflected lightoutput of the pattern image P1 in the case where the irradiation lightamount L2 is used. The process from the latter half of the procedure(a1) to the procedure (c1) is equivalent to the step S103 in FIG. 4.

Procedure (d1): The CPU 51 performs the gradation correction control onthe basis of the calculated density value of the pattern image P1.

The gradation correction control of the procedure (d1) is equivalent tothe step S104 in FIG. 4. First, the CPU 51 generates the correction dataon the basis of the calculated density value of the pattern image P1,and the output gamma correction module 509 corrects the input image datausing the correction data (γLUT). Next, the γLUT that is updated by themeasurement result of the density of the pattern image P1 will bedescribed with reference to FIG. 11.

FIG. 11 is a graph showing an example of the γLUT stored in the RAM 53.Although only the gradation correction process for a cyan image will bedescribed, magenta, yellow, and black images will be corrected by thesimilar method. The γLUT is correction data for correcting the inputimage data so that the density of the input image data linearly relatesto the density of the output image. A horizontal axis denotes inputimage data, and a vertical axis denotes the measured density value(determined density value DENS(i)) of the pattern image P1 that ismeasured with the pattern sensor 41.

Moreover, a linear target gradation characteristic TARGET indicates agradation characteristic as a target of the image density control. Thepoints C1 through C8 indicate the measured density values of the cyanmeasurement images of the pattern image P1. A curve r connects themeasured density values of the pattern image P1. In the description, thecurve r indicates a gradation characteristic before performing the imagedensity control. It should be noted that density values of gradationsthat are not included in the pattern image on the curve r are calculatedby performing spline interpolation so that the curve r connects anorigin and the points C1 through C8. A curve D indicates the γLUTcalculated by the image density control. The curve D is calculated byfinding symmetrical points to the target gradation characteristic TARGETof the curve r before the correction. When the measured density value iscorrected on the basis of the curve D (when the value on the curve D ismultiplied to the density value of the input image, for example), thegradation characteristic of the density value of the output image to thedensity value of the inputted image approaches to the target gradationcharacteristic TARGET.

When the γLUT (curve D) calculated (generated) is stored into the RAM53, it is updated by being replaced with the γLUT generated beforehand.After that, the image forming apparatus obtains an image of the targetdensity by correcting the input image data using the updated γLUT and byforming the image according to the corrected image data.

FIG. 12A is a view showing the measurement accuracies in a case wheremeasurement images of various gradations are measured with the lowirradiation light amount L1 in comparison with the measurementaccuracies in a case where the measurement images are measured with thehigh irradiation light amount L2. In this example, the irradiation lightamount L2 is twice the irradiation light amount L1.

FIG. 12B is a view showing the measurement accuracies of the measurementimages of various gradations of the pattern image in the firstembodiment in comparison with the measurement accuracies of themeasurement images of various gradations of the pattern image with thelow irradiation light amount L1. In the first embodiment, themeasurement images of the low gradation of the pattern image aremeasured with the irradiation light amount L1, and the measurementimages of the high gradation is measured by the irradiation light amountL2. Since the measurement images of the low gradation of the patternimage are measured with the irradiation light amount L1 in both thecases, there is no difference in the measurement accuracy. On the otherhand, concerning the measurement images of the high gradation, themeasurement accuracy in the first embodiment measured with theirradiation light amount L2 becomes about twice the case measured withthe irradiation light amount L1.

As described above, since the measurement images of the high gradationof the pattern image are measured with the high irradiation lightamount, variation of the sensor output in response to the variation ofgradation of the pattern image becomes large, and an error at the timeof converting the density becomes small. Moreover, effects of errorsthat do not depend on the light amount, such as electric noise of thelight receiver, also become small relatively.

According to the first embodiment, the first profile (first surfacedata) is obtained by the actual measurement as the one-round profilethat is used in the determination of the density. On the other hand, thesecond profile (second surface data) is obtained by calculation on thebasis of the first profile and the irradiation light amounts L1 and L2.Then, the irradiation light amount used in the determination of thedensity is set up corresponding to the gradation of the pattern image.That is, the CPU 51 determines the density value DENS(i) about themeasurement images of the pattern image P1 in the first gradation rangeon the basis of the first profile and the reflected light output of thepattern image P1 in the case where the irradiation light amount L1 isused. Moreover, the CPU 51 determines the density value DENS(i) aboutthe measurement images of the pattern image P1 in the second gradationrange on the basis of the second profile and the reflected light outputof the pattern image P1 in the case where the irradiation light amountL2 is used. This enhances the density determination accuracy of thepattern image in a wide gradation range. Accordingly, the irradiationlight amount is set up appropriately so as to enhance the measurementaccuracy (density determination accuracy) of the pattern image, whichimproves the image quality of a printed image.

Moreover, the gradation range of the measurement images of the patternimage P1 that are measured with the irradiation light amount L1 (thefirst gradation range) includes the gradation in which the output of thepattern sensor 41 is saturated while using the irradiation light amountL2. This prevents saturation of the output in measurement of themeasurement images of all the gradations of the pattern image P1.

Moreover, the second profile is calculated according to the formula (3)using the sensor output in a case where the irradiation light amount is0 as a standard. Thereby, the second surface data is calculatedcorrectly by eliminating an effect of the dark current voltage.

It should be noted that a period may be needed until an actualirradiation light amount is stabilized depending on a type and lightamount setting of the pattern sensor 41. Accordingly, the CPU 51 mayprovide a predetermined time interval at time of switching theirradiation light amount in order to measure the pattern image P1 withthe stable irradiation light amount. For the purpose, the measurementimages may be spaced apart from each other in an area where theirradiation light amount is switched so as not to use an area in whichthe light amount is not stabilized, for example. That is, apredetermined space is given between the low-density measurement imagesand high-density measurement images of the pattern image P1, and aswitching timing of the light amount is matched with the space.Alternatively, a predetermined time interval may be given by changingbelt conveyance speed on the way while forming the measurement images ofthe pattern image P1 regularly. For example, the CPU 51 lowers the beltconveyance speed.

Although two stages of irradiation light amounts are set up for thedetermination of the density corresponding to the gradations of thepattern image, three or more stages may be set up.

Next, a second embodiment of the present invention will be described. Asmentioned above, the density correction control is roughly divided intothe Dmax control and the gradation correction control. The gradationcorrection control in the first embodiment particularly enhances thereading accuracy of the high-density measurement images by changing theirradiation light amount between the time of measurement of thelow-density measurement images and the time of measurement of thehigh-density measurement images of the pattern image P1, Against this,the second embodiment of the present invention switches the irradiationlight amount used at the time of measurement of the pattern imagebetween control to determine a development contrast (the Dmax control)and the gradation correction control. The development contrast iselectric potential difference between the exposure electric potential ofan image bearing member and developing bias. The second embodiment willbe described with reference to FIG. 13 through FIG. 16 in addition tothe first embodiment.

FIG. 13 is a view showing an example of a pattern image P2 fordevelopment-contrast correction used in the Dmax control. The patternimage P2 is formed on the intermediate transfer belt 27 in the secondembodiment. It should be noted that the pattern image P2 is enough to beformed on an image bearing member, and may be formed on thephotosensitive drum 22Y, 22M, 22C, or 22K. For example, when the patternsensor 41 measures a pattern image formed on the photosensitive drum22Y, for example, the pattern sensor 41 is enough to be arrangedopposite to the photosensitive drum 22Y on which measurement images areformed.

An arrow in FIG. 13 indicates the rotational direction of theintermediate transfer belt 27. The pattern image P2 is a group ofmeasurement images each of which is a square with a one-side of 25 mm.Five measurement images are formed for each of Y, M, C, and K in thepattern image P2 while changing the development contrast in five stepsof V1, V2, V3, V4, and V5 by changing charging bias, developing bias, alaser exposure intensity, etc. The pattern image P2 consists of thetwenty measurement images in total that are formed in the rotativedirection (circumferential direction) of the intermediate transfer belt27. It should be noted that an image printing rate (density gradation)of all the measurement images of the pattern image P2 shall beidentical.

FIG. 14 is a flowchart showing a process of density correction controlexecuted by the image forming apparatus according to the secondembodiment. The process of this flowchart is achieved when a programstored in the ROM 51 is developed to the RAM 53 and the CPU 51 runs theprogram.

It should be noted that the first profile (one-round profile of thereflected light output L1B(i)) and the second profile (one-round profileof reflected light output L2B(i)) are obtained also in the secondembodiment as with the first embodiment (FIG. 9). Moreover, the meaningof the irradiation light amounts L1 and L2 are the same as thatdescribed in the first embodiment.

The CPU 51 determines whether the Dmax control is required in step S301.When a cumulative amount of image forming operations is more than apredetermined amount, or when a user designates an execution, forexample, it is determined that the Dmax control is required. When theDmax control is not required, the CPU 51 proceeds with the process tostep S304. On the other hand, when the Dmax control is required, theprocess proceeds to step S302.

In the step S302, the CPU 51 sets the irradiation light amount that isused for the measurement of the pattern image P2 in the pattern sensor41 to L2. Since the Dmax control is performed so that the maximumdensity in which the image printing rate is 100% becomes a targetdensity, a high-density pattern image is used. It is appropriate thatthe high-density pattern image is measured with the high irradiationlight amount L2 so as to enhance the measurement accuracy. Next, the CPU51 performs the Dmax control in step S303. Details of the Dmax controlwill be mentioned later as procedures (a2) through (d2).

In the step S304, the CPU 51 sets the irradiation light amount to L1.This enables the measurement of the pattern image P1 that consists ofthe measurement images in the low-density range and high-density rangeby the gradation correction control performed thereafter. Next, the CPU51 performs the gradation correction control in step S305. The gradationcorrection control is performed after the Dmax control in the step S303,because the execution of the Dmax control changes the gradationcharacteristic by the correction of the development contrast. In thegradation correction control, the measurement images of the patternimage P1 of all the gradations are measured with the same irradiationlight amount L1. It should be noted that the irradiation light amountused for determining the density may be set according to the gradationof the pattern image P1 also in the gradation correction control asdescribed in the first embodiment. Then, the process in FIG. 14finishes.

Incidentally, the development contrasts of the measurement images in thepattern image P2 used for the Dmax control are set up as follows.

DmaxY1, DmaxM1, DmaxC1, DmaxK1:V1

DmaxY2, DmaxM2, DmaxC2, DmaxK2:V2

DmaxY3, DmaxM3, DmaxC3, DmaxK3:V3

DmaxY4, DmaxM4, DmaxC4, DmaxK4:V4

DmaxY5, DmaxM5, DmaxC5, DmaxK5:V5

Next, a concrete example of the Dmax control process in the step S303will be described. The Dmax control process is performed along thefollowing procedures (a2) through (d2).

Procedure (a2): When the Dmax control is performed, the CPU 51 forms thepattern image P2 on the intermediate transfer belt 27, and measures thepattern image P2 with the pattern sensor 41 while controlling thepattern sensor 41 with the irradiation light amount L2.

Procedure (b2): The CPU 51 specifies the reflected light output of theposition on the intermediate transfer belt 27 corresponding to theformation position of the pattern image P2 on the basis of theirradiation light amount L2 at the time of measuring the pattern imageand the formation position of the pattern image P2. In this time, theCPU 51 specifies the reflected light output of the position on theintermediate transfer belt 27 corresponding to each of the formationpositions from the second profile.

Procedure (c2): The CPU 51 determines the density of the pattern imageP2 using the reflected light output of the pattern image P2 and thereflected light output of the intermediate transfer belt 27.Specifically, the CPU 51 calculates (determines) the density valueDENS(i) about the pattern image P2 on the basis of the value obtainedfrom the second profile and the reflected light output of the patternimage P2 in the case where the irradiation light amount L2 is used.

Procedure (d2): The CPU 51 performs a development contrast control onthe basis of the calculated density value of the pattern image P2.

The contrast control of the procedure (d2) is performed as follows. FIG.15 is a graph showing a relation between the density value of thepattern image P2 and the development contrast. The relation is stored inthe ROM 52. The CPU 51 determines the development contrast at which theimage formation is performed on the basis of the density value of thepattern image P2. That is, the CPU 51 determines the developmentcontrast so that the maximum density in which the image printing rate is100% becomes a target density using the relation shown in FIG. 15. Thedetermined development contrast is reflected to subsequent imageformations.

FIG. 16 is a view showing comparison of the measurement accuracies incases where the pattern image of the same gradation is measured with theirradiation light amounts L1 and L2. As compared with the measurementwith the irradiation light amount L1, the accuracy of the measurementwith the irradiation light amount L2 in the second embodiment is high.

According to the second embodiment, the CPU 51 differentiates theirradiation light amount between the case where the gradation iscontrolled and the case where the development contrast is controlled.When controlling the development contrast, the CPU 51 determines thedensity of the pattern image P2 for correcting the development contraston the basis of the second profile and the reflected light output of thepattern image P2 in a case where the irradiation light amount L2 isused. This enhances the determination accuracy of the density of thepattern image for correcting the development contrast. Accordingly, theirradiation light amount is set up appropriately, which enhances themeasurement accuracy of a pattern image and improves the quality of theprinted image in the same manner as the first embodiment.

Next, a third embodiment of the present invention will be described. Animage forming apparatus 100 according to the third embodiment uses theirradiation light amount L1 in a case where pattern images of chromaticcolors (yellow, cyan, and magenta) are measured and uses the irradiationlight amount L2 in a case where a black pattern image is measured. Sincea reflected light amount from a black pattern image is less than thatfrom a pattern image of a chromatic color, there is high probabilitythat a sensor output of the black pattern image lowers. Since the imageforming apparatus 100 according to the third embodiment uses theirradiation light amount L2 for measuring the black pattern image,degradation of the sensor output is reduced and the pattern image isdetectable with high accuracy.

Other Embodiments

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2017-024015, filed Feb. 13, 2017, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An image forming apparatus that forms an image ona sheet, the image forming apparatus comprising: an image bearingmember; an image forming unit configured to form an image on the imagebearing member; a light emission unit; a measurement unit configured tomeasure reflected light from a measurement image formed on the imagebearing member; a controller configured to control the image formingunit to form measurement images, to control the light emission unit toemit light, to control the measurement unit to measure reflected lightfrom the measurement images, and to generate an image formationcondition based on measurement results of the measurement images andinformation related to a measurement result of the image bearing member,wherein the controller controls the light emission unit to emit lightbased on a first measurement condition, and controls the measuring unitto measure the reflected light from the image bearing member, anddetermines first information corresponding to the first measurementcondition based on the measurement result of the image bearing member,wherein the controller determines second information corresponding to asecond measurement condition based on the first measurement condition,the first information, and the second measurement condition, whereinlight intensity corresponding to the second measurement condition ismore than light intensity corresponding to the first measurementcondition, wherein the controller controls the light emission unit toemit light based on the first measurement condition in a case where themeasurement unit measures a first measurement image based on the firstmeasurement condition, and generates the image forming condition basedon a measurement result of the first measurement image and the firstinformation, and wherein the controller controls the light emission unitto emit light based on the second measurement condition in a case wherethe measurement unit measures a second measurement image based on thesecond measurement condition, and generates the image forming conditionbased on a measurement result of the second measurement image and thesecond information.
 2. The image forming apparatus according to claim 1,wherein the controller controls the image forming unit to form aplurality of measurement images of which gradations are different, andwherein the controller selects a measurement condition based on thegradation.
 3. The image forming apparatus according to claim 1, whereinthe image forming unit forms the first measurement image correspondingto a first gradation range, and wherein the image forming unit forms thesecond measurement image corresponding to a second gradation range thatis higher than the first gradation range.
 4. The image forming apparatusaccording to claim 1, wherein the controller generates a first imageforming condition for correcting a gradation characteristic of an imagethat the image forming unit will form based on the measurement result ofthe first measurement image and the first information, and wherein thecontroller generates a second image forming condition for adjustingmaximum density of an image that the image forming unit will form basedon the measurement result of the second measurement image and the secondinformation.
 5. The image forming apparatus according to claim 1,wherein the controller controls a signal value input into the lightemission unit in order to adjust a measurement condition.
 6. The imageforming apparatus according to claim 1, wherein the controllerdetermines the first information based on one-round measurement resultof the image bearing member.
 7. The image forming apparatus according toclaim 1, further comprising a sensor that detects a mark attached to theimage bearing member, wherein the controller determines the firstinformation based on a detection result of the mark by the sensor. 8.The image forming apparatus according to claim 1, further comprising aconversion unit configured to convert image data based on a conversioncondition, wherein the image forming condition corresponds to theconversion condition.
 9. The image forming apparatus according to claim1, wherein the controller divides the measurement result of the firstmeasurement image by the first information, and generates the imageforming condition based on a calculated result, and wherein thecontroller divides the measurement result of the second measurementimage by the second information, and generates the image formingcondition based on a calculated result.
 10. A control method for animage forming apparatus that forms an image on a sheet, the controlmethod comprising: controlling an image forming unit to form ameasurement image on an image bearing member; controlling a lightemission unit to emit light; controlling a measurement unit to measurereflected light from the measurement image; generating an image formingcondition based on a measurement result of the measurement image andinformation related to a measurement result of the image bearing member,wherein the measurement unit measures light emitted from the lightemission unit based on a first measurement condition and is reflectedfrom the image bearing member, and first information corresponding tothe first measurement condition is generated based on the measurementresult of the image bearing member, wherein second informationcorresponding to a second measurement condition is determined based onthe first measurement condition, the first information, and the secondmeasurement condition, wherein light intensity corresponding to thesecond measurement condition is more than light intensity correspondingto the first measurement condition, wherein the light emission unitemits light based on the first measurement condition in a case where themeasurement unit measures a first measurement image based on the firstmeasurement condition, and the image forming condition is generatedbased on a measurement result of the first measurement image and thefirst information, and wherein the light emission unit emits light basedon the second measurement condition in a case where the measurement unitmeasures a second measurement image based on the second measurementcondition, and the image forming condition is generated based on ameasurement result of the second measurement image and the secondinformation.